You'll need to check with your municipalitySome additional food for thought:Lot splits in my area take a lot of time, patience and moneyYou can finance the construction of an owner-occupied 2-4 units with an FHA loan for as little as 3.5% down but the requirements are kind of onerous. 

In addition to finding out the values of other properties in the area, the only way you'll be able to get an 80% offer accepted is if the property needs some work or is distressed to some degree.

In addition, employer contributions may be withdrawn upon the occurrence of any of the following events:1) The occurrence of a Hardship,2) The attainment of age 59 ½3) The employer contributions being withdrawn have been accumulated in the solo 401k plan for at least 2 years; or4) The participant has participated in the solo 401k plan for at least 5 years.Salary Deferrals(employee contributions): Any employee contribution (including any earnings on such amounts)may not be distributed prior to the solo 401k participant’s severance from employment, death, or disability.

Additional comments to help answer questions you may have: She has never said whether she intends to rent the home in the name of a company, nor has she offered any financial info on any potential guarantor.

There are other miscellaneous expenses that will occur that can add up to an additional 5% or more.  

Through the program, the insulation is 90% off, and then I can take an additional $750 off for a knob / tube inspection.

It isn't that i'm not curious what the appraisal would come in at, but I will likely be getting one when i go to refinance in 6-18 months and all the value would do right now is: 1) Give me a more accurate idea of what the bank would value the property at2) Delay closing as appraisals are 3 weeks out minimum right now3) If it comes in high the seller could use that info to back out4) Add additional costs (around $950) to the closingP.s. - I will still be ordering an inspection.Thanks in advance!

If you can really get $325 per night and have decent occupancy, you can use the extra AirBNB income to make additional payments on your primary loan, to slowly reduce the interest and timeframe.

The only portion you should figure the additional cost at 3.75% vs 5.25% is the principle loan amount you have now. 

In addition, solar requires monitoring to ensure that it is working fine and the solar panels need cleaning every few months.